Commonly people feel that "every human is equally valuable" (in terms of
moral utility).
Thus, the utility of your actions scales linearly with the number of human
lives you save, or the number of humans you benefit (with constant benefit
per person).

However, the question is whether this intuition continues, so that the limit
of the utility function, as the number of humans approaches infinity, is
infinite.

In fact, De Blanc says (to simplify greatly), your utility function must be
bounded from above.